Prop Slip ?

[ril0]

have fitted merc 150hp seems like prop is slipping but have tried three different props with same result .When revs get to 1500rpm prop seems to loose grip and revs rise with no increase in speed . Engine install is correct and dont think can be cavitation as revs not high enough .Is there a clutch or anything in gear box that could slip reverse is fine and there dosnt seem to be any grateing or grinding noises .Also drive feels smooth with no jumping or clunking when selecting gears . Just found out prop I am using is fromm merc inline six there is gap around rear of prop between gear case and prop would this give above symptoms. Dont want to buy new prop if dont need to.

 

[WillyBWright]

Re: Prop Slip ?

It sounds like you're using small hub props on a large hub gearcase? It depends on the boat if that will work. Speed and bass boats usually tolerate it. Big cruisers don't. What's happening is that exhaust bubbles are blowing over the blades and you're pushing foam instead of water. On certain boats, this is advantageous because it allows the motor RPMs to rise quickly which provides more power to get out of the hole. When the boat planes, all of the exhaust is drawn through the center of the prop and the prop "hooks-up". Somtimes it feels like a 2nd gear kicks in. But if the boat is too heavy, it doesn't get on plane and all you're pushing is foam.

The above is mostly true with stainless props. Small hub aluminium props don't usually work well on large hub gearcases no matter what boat.

 

[Crownie2]

Re: Prop Slip ?

I guess we assume all three props were the same style (with the gap)??

If so, I'd suggest you break down and buy a new prop...

 

[ril0]

Re: Prop Slip ?

Ok thanks Willy have now ordered new 22inch pitch prop for motor .My boat is 23ft long and is a deep v hull so weighs about 1.5 tons so sounds like that could be the problem. Just hope got the pitch right
Cheersd

 

[ril0]

Re: Prop Slip ?

Yeah thanks Cronnie they were all the same props have got new one now see last post
Cheersd

 

[walleyehed]

Re: Prop Slip ?

How did you come up with the 22 pitch as being the pitch you need?

 

[ril0]

Re: Prop Slip ?

yeah when running with 115 inline had 17 pitch so had a bit of guess also mate of mine got similar setup with 22 pitch do you know of a formula I could use

 

[walleyehed]

Re: Prop Slip ?

Which 150 do you have?

 

[ril0]

Re: Prop Slip ?

merc 150 black max .Its the 2 litre v6 approx mid 80s

 

[ril0]

Re: Prop Slip ?

Tried new prop today ran fine ,although could do with smaller pitch max out at 4500 revs at 29kts and took long time to get out of the hole .Maybe try 19inch pitch should get about right revs and hopefully a bit more top end speed .Any ideas ?

 

[walleyehed]

Re: Prop Slip ?

That'll put you in the 5100-5200RPM range...Just what RPM did you have with the 17?

 

[ril0]

Re: Prop Slip ?

the 17 was from merc 115 inline 6 and caused prop slip so couldnt tell .do you think te 19 would be ok at 5100-5200 revs or maybe a 17 perhaps bringing revs to max 5400-5600 bearing in mind its an old engine .Im thinking it would be wiser to keep the revs down a bit.

 

[walleyehed]

Re: Prop Slip ?

The older engine needs just the opposite...it needs higher RPM. today's fuel doesn't do as well in the older engines, but we've found better longevity and cleaner engines if we run'em a bit higher RPM-wise.
I never thought about it at first, but when going from the small gearcases and that ratio, to the large Gearcase in the HP you did, the prop size in pitch will be about the same...The V6 is spinning a larger Blade area prop in the same pitch, much faster than the 115, so they will remain close to the same, pitch-wise.

 

[ril0]

Re: Prop Slip ?

ok thanks wallyehed like you said when you think about it that would be correct cheers for the advise .

 

[dmarkvid2]

Re: Prop Slip ?

do you use trim tabs on your boat?

 

[Crownie2]

Re: Prop Slip ?

Iain - I had an '85 175 Mariner and it really liked to run at around 5,800 - 5,900 rpm's.

For your 150 I'd say about 5,500 -5,600 would be about right...

Bob

 

[ril0]

Re: Prop Slip ?

No trim tabs just engine tilt trim .Cheers Bob will keeparound those revs then .What speed you reckon I guess around 40mph

 

[Crownie2]

Re: Prop Slip ?

I'm guessing the 17" should be about right and 40 is a very good estimate, IMO.

Bob